In the art of earth-orbiting satellites it is known to establish a communication link between an orbiting satellite and an earth monitoring and control station, or "control segment". Such a communication link may be used for downloading information which is critical to the monitoring of the satellite performance and for uploading commands necessary for the proper performance, and to correct defects in that performance. The determination of satellite commands usually requires extensive dam analysis utilizing complex computer programs, which is labor intensive. In systems which require that the satellite computer predict accurate current and future position with respect to the earth, a significant part of the control segment data analysis is used to determine the position prediction parameters to send to the satellite, and a significant portion of the total control segment transmission of data to the satellite is allocated the transmission of these position prediction parameters. The anchor concept uses a combination of computations performed on the satellites and a fixed satellite or simulated satellite of known position with respect to the Earth to anchor the constellation and significantly reduce control segment work load.
The anchor concept may be applied in systems requiring the satellite computer to predict accurate current position with respect to the earth. For example, the anchor concept may be applied in systems requiring the exchange of navigational and clock data between a constellation of earth-orbiting satellites and a control segment located on the earth surface. Application of the anchor concept in this type of system affords the advantage of a stable coordinate reference with significant reductions in the data analysis, the accompanying man hours, and in the data transmission up to the satellite, thereby significantly reducing the control segment work load.
The orbital path of the satellite is known, either from data from the control segment, or from the exchange of navigational and clock data in a constellation of satellites with processing similar to that known as autonavigation "AUTONAV" on the GPS global positioning satellite system). Since the anchor's position is fixed and known relative to the earth's surface, a prediction of the relative positions of the satellite to the anchor and of the satellite to the earth's surface can be made. This can be readily accomplished without the need for human intervention. System integrity is also improved by reducing the amount of data transmitted to the satellite with its inherent chance of error and the chance of human error. The result is a significant reduction of system life cycle cost.
The anchor's fixed position relative to the earth makes it possible to accurately compute the position of an orbiting satellite relative to the anchor. This is particularly important in satellite systems that cannot function properly without knowledge of satellite position relative to the earth's surface. Typical of such a system would be any communication network that proposes to provide global point-to-point coverage through the use of earth-orbiting satellites. In order for such a system to be operable, it must be capable of determining if the satellite selected to relay the communication is "visible" to both points on the earth, or if a satellite-to-satellite relay is required because the source and the destination are not visible from the same satellite. The visibility of the satellite to any point on the earth is a function of the position of the point, the position of the satellite, and the beamwidth of the satellite's antenna. Since the position of a point on the earth's surface and the beamwidth of the satellite's antenna are known, the only remaining unknown necessary to a visibility determination is the satellite position. The anchor provides a way to determine this satellite position.
The use of an anchor to determine the position of a global communication satellite with respect to the earth is one example of how the anchor concept may be applied. Another example of the anchor's utility is found in its application to global navigation systems.
The preeminent global navigation system in use today is the Global Positioning System (GPS). The GPS is composed of a plurality of satellites orbiting at approximately 11,000 nautical miles above the earth and maintained in almost perfectly circular orbits. These orbits are chosen so that the system can provide information to a user regardless of the time that the user requests information and regardless of the user's position on the earth's surface. A user of the GPS determines his latitude, longitude, and altitude by employing a GPS receiver. The GPS receiver engages in a radio-ranging calculation with the GPS satellites and then employs a three dimensional equivalent of the traditional "triangulation" technique on the data it receives from the satellites to compute the user's position. In order to use this "triangular" technique, four of the orbiting GPS satellites must be "visible" to the user at any one time. Also, the position of these four satellites relative to the earth must be known--the greater the accuracy of the reported satellite position, the greater the accuracy in the user position determination. Therefore, a stable and precise reference that can provide for an accurate determination of satellite positions relative to the earth's surface is a critical element of the GPS. Without it, the GPS could not carry out its primary task of providing users with accurate position information. Currently, this reference is supplied manually by the Control Segment.
The method used to compute the range to a GPS satellite involves the transmission by each satellite of encoded pulses of electromagnetic energy. The pulses will be incident upon the GPS receiver after a delay that is proportional to the distance from the satellite to the receiver. The pulses are then decoded by the receiver to determine the identity of the transmitting satellite, the time of transmission, and the position of the satellite at the time of transmission. When four such pulse sequences are transmitted closely in time, one by each of four different satellites, the receiver can calculate the average GPS Time and its range to each of the four satellites at the time of transmission and, from those ranges and the known position of the satellites, it may then calculate its position relative to the earth.
For the GPS ranging method to yield an accurate determination of satellite-to-receiver range, the timing reference aboard each satellite must be in synchronization with the receiver time reference. The satellite time references are synchronized by the use of the "AUTONAV" process in GPS block IIR, which is well known to those knowledgeable in GPS technology. Once the satellite time references and receiver time reference are locked to a standard time reference, and the use of the fourth satellite ranging pulse sequences allow the GPS receiver to synchronize, accurate range measurements may be achieved.
The anchor provides a way to synchronize the satellite time reference, called "GPS time", to an earth standard time reference known as UT1. In addition, since the range to the satellite is measured and the position of the anchor is known, the information necessary for the satellite to determine the difference between the measured range and the estimated range is used as a measurement in the satellite to correct the satellite's estimate of its current trajectory parameters.
In addition to the anchor's GPS functions of determining satellite position and monitoring the satellite timing references, another important anchor function is that of maintaining the GPS satellites in their proper orbits. Any deviation of a satellite from its prescribed orbit will adversely affect the accuracy of predictions of that satellite's position, and in the extreme case could result in the loss of the satellite. For these reasons it is important to detect errors in the satellite's orbit and correct them when necessary. The anchor may be used to sense errors in the satellite's orbit and to issue change of position, or "delta-v", velocity correction commands to re-position the satellites, or it could be used to check a control segment delta-v command to improve integrity.
Maintenance of the satellite's orbits requires that the control segment accurately track the position of the satellites before, during, and after the implementation of a delta-v command. In the current GPS configuration, this tracking function is carried out through the use of a Kalman Filtering Algorithm.
In a Kalman Filtering Algorithm, a Kalman Filter or mathematical filter/predictor is provided with satellite position as reported from two different sources, from which the filter generates a satellite position determination that is more accurate than could be achieved by either source individually. One of the filter's sources provides updated predictions of the satellite positions as determined by a mathematical model of the satellite orbit. The second source provides updated satellite position data as communicated from the satellites themselves, i.e., each satellite's position as computed by its internal tracking mechanism. After receiving information from the two sources, the Kalman Filter computes a new position determination based on a weighted average of the received information. In this manner the Kalman Algorithm can operate to minimize the mean-squared error among the satellite position determinations. A detailed description of how a Kalman Algorithm minimizes the mean-squared error inherent in tracking a set of data can be found in "Radar Handbook", edited by Merrill Skolnik, 2d edition, published by McGraw Hill, Inc. (1990) (see Chapter 8, entitled "Automatic Detection, Tracking, and Sensor Integration" by G. V. Trunk). Also, in the article entitled "Sensitivity Analysis of an Integrated NAVSTAR GPS/INS Navigation System to Component Failure" by H. M. Schwartz, published in Journal of the Institute of Navigation, Vol. 30, No. 4, winter 1983-1984, pp. 325-337, two examples are given of how position coordinates originating from various sources are mixed by means of a Kalman Filter so as to generate one system of position coordinates, the accuracy of which is substantially greater than that of the individually presented position coordinates.
The Kalman Filter solution for position determination will, in most instances, represent the best available solution for position and orbit of the satellites. However, it may diverge from the correct solution in some cases. The Filter requires an initial estimate of the satellite positions to become operational. These initial estimates give rise to a filter transient response. Accordingly, the Kalman Filter solution may fluctuate widely during initial operation, and then converge on the correct solution for satellite position as the period of transient response decays. A similar transient response results whenever the "initial" estimates are changed, such as when a delta-v maneuver is executed. Following a delta-v maneuver, the Filter is reinitialized, thereby causing a transient response and rendering the GPS constellation unstable for the period of the response. Moreover, if presented with bad initialization data, the Kalman Filter solution may diverge entirely from the correct solution.
Once the control segment has arrived at a viable Kalman Filter solution, it must communicate the solution to the satellites so that the satellites may, in turn, pass on accurate positional data to GPS users. The operation of uploading current satellite data and downloading satellite information takes about 20 minutes. These operations can only be performed during the period in which the satellite being communicated with is visible to the control segment--a minimum period of about one hour per pass is required for each satellite. Since the required communications take about 20 minutes, and another 20 minutes is required for safety retransmission, this leaves only 20 minutes left for establishing communications. The limited period for establishing communications can present a problem, particularly when operational and transmission errors occur. If the required communications can not be completed in a visibility period, the satellite and control segment must function on old data, a situation which results in an overall decrease in positional accuracy.
Further complicating the satellite-control segment communications is the quantity of data that must be transmitted. In addition to uploading current satellite position, a prediction of future satellite position is uploaded. The position of a current GPS or a block IIA satellite is predicted for a period of 14 days into the future. In this manner, the satellite will be able to use predicted positional data in lieu of actual positional data from the control segment, thus keeping the satellite operational despite any loss of the control segment. The 14 days corresponds to the maximum amount of time that the satellite will be able to remain operational following an event that results in the loss of communications from the control segment. Of course the use of predicted data introduces an additional source of error into the system, and thereby results in less accurate user data during periods of control segment unavailability.
Efforts have been made to reduce the satellites' dependence on the control segment. Most notably, the newer generation of satellites, or "GPS block IIR" satellites have been provided with an autonomous navigation capability, or "autonav" and a prediction time of 210 days instead of 14 days.
To autonomously navigate, each of the GPS block IIR satellites implements its own Kalman procedure. At periodic intervals, each block IIR satellite broadcasts its clock value, ephemerous data, and Kalman Filter data--collectively known as the "autonomous navigation message". This data is used by all receiving satellites to update their own Kalman procedures. In order for satellites to broadcast and receive data among themselves they are equipped with transmitters and receivers dedicated to that purpose, collectively known as the satellite "crosslinks". All block IIR satellites that are in view of the broadcasting satellite's antenna will receive broadcasted data. A broadcast period is defined, and each satellite broadcasts during an assigned portion of that period. Thus, the identity of broadcasting satellite can be determined by any receiving satellite merely by noting the time of broadcast, although this is not the only method of determining the broadcasting satellite. This method of determining a broadcasting satellite's identity is sometimes referred to as "Time Division Multiplexing" (TDM). The range between satellites is computed by noting the elapsed time between transmission by the broadcasting satellite and reception by the receiving satellite and then multiplying that time by the speed of propagation, nominally the speed of light. Currently, through crosslink communication, block IIR satellites can maintain highly accurate determinations of their position relative to one another; however, determinations of their position relative to the surface of the earth are still subject to the limitation associated with control segment processing. For a more in depth discussion of the basic design and functions of the GPS block IIR satellites and their autonomous navigation function see: "An AUTODOP Algorithm for the Block IIR Autonav Capability" by Alison Brown, Institute of Navigation Satellite Division, Technical Meeting, 2d, Colorado Springs, Colo., September 27-29, Proceedings, Washington, D.C., Institute of Navigation, 1989, pp. 291-293.
The crosslink communication between satellites and the communication between a satellite and the control segment are illustrated in FIGS. 1 and 2. In FIG. 1 the communication between a broadcasting satellite 2 and two satellites 4 and 6 that are visible to the broadcasting satellite antenna is shown. Also shown in FIG. 1 is the communication between a satellite 2 and a control segment 8. In the current GPS configuration, satellite to control segment transmissions are accomplished using both S and L band radio frequencies while control segment to satellite communications are accomplished using S-band radio frequencies. FIG. 2 is a depiction of how clock value, ephemerous data, and Kalman Filter data is "indirectly communicated" from a broadcasting satellite 10 to an invisible satellite 12. In FIG. 2 a third satellite 14 receives data from the broadcasting satellite 10. This data is used by the third satellite 14 to update its clock, ephemerous data, and Kalman Filter data. Thus, when the third satellite 14 broadcasts, it is "indirectly communicating" the broadcasting, processed satellite 10 data to the invisible satellite 12. The communication between satellites 10, 12 and 14 occurs regardless of whether the control segment 8 is visible to the satellites.
As noted above, each block IIR satellite has an on-board Kalman Filter that the satellite uses to generate a solution for the satellite positions. Using these Filters in conjunction with periodic receptions of data from the other satellites, each satellite can maintain an accurate fix on this position relative to all other satellites, but communication with the control segment is still necessary in order for the satellite to maintain a fix of their positions relative to the surface of the earth, as well as to update ephemerous prediction data. Through the use of autonav, control segment--satellite communications can be made less frequently without a dropoff in the accuracy of satellite position relative to the earth's surface. Thus, in the event that the control segment can not be communicated with (e.g. it has been damaged or destroyed by an act of nature or war), the block IIR satellites can continue to operate within a specified error tolerance for a longer period of time than non-autonav satellites under the same circumstances. This characteristic of the block IIR satellites maintains block IIR operation for a 210 day period of control segment unavailability.
A further advantage of autonav is the small time constant of the block IIR Kalman Filters as compared to the time constant of the control segment Kalman Filter. Because of their smaller time constant and faster sampling, the block IIR Kalman Filters can respond more quickly to velocity changes or delta-v maneuvers. The quicker response reduces the transient response caused by a delta-v maneuver and, in turn, reduces the period of unavailability that follows such maneuvers.
It is one object of the present invention to enhance the performance of the GPS by reducing the current GPS control segment workload using an "autonavigation anchor"--that is, a portion of the control segment that communicates with the block IIR satellites in essentially the same manner that those satellites communicate with each other.
An autonavigation anchor will broadcast and receive data similar to autonav the way a block IIR satellite does. While the anchor may be located on the earth's surface, such location is not essential. Any position that is known relative to the earth's surface will suffice. For example, the anchor may take the form of a geosynchronous satellite. In an autonavigation anchor system, the block IIR satellites will know their position relative to the earth's surface because they know their positions relative to all autonav satellites, and the anchor would be, in effect, another autonav satellite, albeit in a known position relative to the earth's surface. Also, the large transient response of the control segment Kalman Filter will be eliminated. Delta-v maneuvers would give rise to the smaller autonav transient response due to the faster sampling rate, thereby decreasing the amount of time that the system is unavailable.
Furthermore, use of an autonavigating anchor will reduce the error associated with limited control segment--satellite visibility (update rate). This follows from the ability of autonav equipped units to "indirectly communicate". By placing the anchor in the autonav "chain", the anchor will receive processed data from invisible satellites through visible satellites.
Moreover, an autonavigation anchor can be used to relay earth standard time referenced corrections to the satellites. Such a reference correction could be relayed to the anchor from an external source located on the earth and then broadcast to the orbiting satellites as part of the anchor's normal autonav message, for example, as a correction of the anchor's own internal determination of the constellation clock value.
By using an autonavigation anchor to perform the clock and position correction normally performed by the control segment, the foregoing advantages may be realized. The advantages will result in superior system performance at a reduced cost and labor associated with operating the GPS, or any other satellite system requiring communication with one or more orbiting satellites.
When the autonavigation anchor is located on the surface of the earth the range error satellite used in its Kalman Filter may be coordinate converted to a measured error which seems to come from a geostationary satellite located above the earth located autonavigation anchor at the mean radius of the constellation. This improves the reaction time of the constellation with respect to a constellation "drift" from estimated position.